Technology for detecting coordinates at any point on a display exists in the form of technology for identifying positions at which light is beamed, and this technology is applied in the field of games devices, and the like. For example, in a game which involves shooting at game characters represented on a display this technology is used to determine whether or not a light ray emitted from a light ray gun has hit a game character, and it identifies the coordinates of the position at which the light ray hits the display surface.
Conventional coordinates detection of this kind makes use of the horizontal synchronizing signal in the video signal generated by the game device. In order to detect coordinates, the game device is provided with a horizontal counter for counting the input number of each picture element in the video signal, a vertical counter for counting the number of scanning lines, and a light ray gun for shooting at characters on the display. The light ray gun is provided with a photosensor which receives light from light points on the scanning lines shown on the display, and which has a certain degree of directionality. In other words, the light ray gun does not emit a light ray, but rather it actually receives a portion of the light emitted from the display.
As the game program is progressing, the game device transmits video signals to the monitor, whilst surveying whether or not a light ray has been emitted from the light ray gun. If the game device determines that a light ray has been fired, the photosensor detects light emitted from the light point on the scanning line when the gun is fired and this detection signal is input to the game device. The game device reads the horizontal counter and vertical counter when the detection signal is input. The scanning position of the light point forming the scanning line is displaced in precise relationship with the synchronizing signals, and therefore the values of each counter when the light point is detected can be set as the coordinate values at the time the gun is fired.
However, when conventional coordinates detection technology has been applied to recent digitalized television devices, it has not been possible to detect coordinates accurately.
In television receivers based on new television formats, such as HDTV, the video signal is stored as digital data and undergoes prescribed signal processing prior to image display. Therefore, a video signal supplied from an external source is first stored in an internal memory in the television receiver, and is then displayed as an image on a receiver tube at a timing which is independent of the synchronization relationship when the signal was supplied. Furthermore, even if a game image is displayed by converting the video signal output from a game device to another image display system,-using a so-called video converter, or the like, there is frequent loss of synchronization between the original video signal and the image shown on the display. As long as the timing at which images are shown on the display is not synchronized with the video signal of the game device, the coordinates obtained by counting the synchronizing signals in the video signal will not match the actual position on the display at which the gun is fired. Moreover, in a standard television, images are displayed by interlaced scanning, but in some display devices, such as those used in personal computers, or the like, images are displayed by sequential scanning, and this difference means that unless the coordinates detecting device identifies whether the scanning method used in the display is interlaced scanning or sequential scanning, it is not possible to perform correct conversion of the coordinates.